Project Summary Obesity is fast becoming a global health pandemic. Characterized by an increase in adipose tissue to the point where it is associated with adverse health effects, the prevalence of obesity has nearly tripled over the past fifty years. Obesity has an enormous economic burden and is the second leading cause of preventable death. The source of increased fat mass in obesity is currently attributed to two mechanisms: adipocyte hypertrophy, the process by which pre-existing fat cells increase in size due to an accumulation of lipids, and adipocyte differentiation from fat precursor cells. Our preliminary studies show that in adult mice, the adipocyte population is capable of substantial replication. In fact, 5% of adipocytes are in growth phase at any time. Furthermore, we find that adipocytes contribute equally to the growth and maintenance of fat tissue. Through three independent experimental approaches, Ki67 labeling, BrdU incorporation, and dilution of an inducible histone2B-green fluorescent protein (H2BGFP) through cell division, we show that adipocytes are capable of replication throughout adulthood. Additionally, we show that adipocyte replication is increased in obese mice. Our overall goal is to better understand the growth and proliferation of white adipose tissue at a cellular level so that we might learn to alter the enlargement of this tissue and provide an effective therapy for obesity. Our main hypothesis is that the proliferation of mature fat cells drives adipose tissue growth and contributes to the development of obesity. To better understand the turnover, replication, maintenance and expansion of adipocytes and the role these cellular processes play in obesity we propose the following specific aims: Aim I. Characterize replication and turnover of adipocytes in normal and modified adipose tissue. Aim II. Genetic manipulation of adipocytes. Aim III. Examine the role of genes linked to lipomatous diseases in adipocyte replication. Ultimately, the goal of these experiments is to 1) measure the adipocyte turnover in adipose tissues, 2) determine the origin of new adipocytes in adipose tissue, 3) examine the cellular processes involved in response to a loss of adipocytes in adipose tissue, 4) understand the consequence of blocking replication of adipocytes in terms of adipose tissue maintenance and growth, and 5) identify key molecules involved in adipocyte replication and to exploit these as possible points of intervention for the treatment of obesity.